function* fetchData() {
    // 性能
    let res1 = yield ajax(url,() => {})
    let res2 = yield ajax(url1,() => {})
    let res3 = yield ajax(url2,() => {})
}
let it = fetchData(); // Generator
let result1 = it.next();
let result2 = it.next();
let result3 = it.next();